The present invention relates to a tire for vehicles provided with a tread having a low rolling resistance.
The invention also relates to a premolded tread, in particular for covering worn tires, having a low rolling resistance, as well as to a method for reducing the rolling resistance of vehicle tires.
As is known, in the field of vehicle tire manufacture, research is increasingly devoted to finding how to reduce as much as possible the rolling resistance of tires.
To this aim, it has been proposed in the art to use rubber compositions having a low content of carbon black in the manufacture of the tire tread.
In these rubber compositions, carbon black is partly or entirely replaced by the so-called xe2x80x9cwhitexe2x80x9d inorganic reinforcing fillers, such as gypsum, talc, kaolin, bentonite, titanium dioxide, various types of silicates, and in particular silica, as disclosed, for instance, in European Patent EP 0 501 227.
In particular, in order to reduce the rolling resistance of the tire without significantly affecting its wear resistance and wet skid resistance, the constant teaching of the prior art in this field is to use silica having a so-called high surface area, i.e. silica having a BET surface area generally greater than 130 m2/g.
According to the constant teaching of the prior art, in fact, the use of silicas having a high surface area allows to improve the mechanical properties of the tread, such as for instance tensile resistance, elongation, tear resistance, and in particular abrasion resistance.
Notwithstanding these advantageous effects, however, the use of silica-based reinforcing fillers involves several drawbacks substantially associated to the poor affinity of the same with the polymer base of the rubber compositions employed in the tire manufacture. Such poor affinity requires, in fact, the use of suitable coupling agents that are able to chemically bond silica to the polymer matrix.
However, the need of using such coupling agents poses a limit to the maximum temperature that may be attained during the steps of mixing and thermal-mechanical working of the rubber composition, on pain of an irreversible thermal degradation of the coupling agent.
But the respect of the aforementioned temperature constraint involves, in its turn, a marked reduction in the very mechanical mixing action that is of the essentialxe2x80x94when using silica having a high surface areaxe2x80x94for an optimum dispersion of the silica in the polymer matrix, as stressed by the aforementioned European Patent EP 0 501 227.
According to the teaching of this patent, in fact, it is possible to achieve a uniform dispersion in the polymer base of the silica having a high surface area only when the rubber composition is submitted to a thermal-mechanical working which is capable to reduce the projected area of silica aggregates from a starting value greater than 8500 nm2, shown by silica per se, to an end value comprised between 7000 and 8400 nm2, after a suitable thermal-mechanical working.
On the other hand, the insufficient dispersion of silica in the rubber composition which would ensue due to an insufficient thermal-mechanical working per se unable to comply with the aforementioned constraint of projected area of the silica aggregates, would cause, in its turn, the worsening of those very mechanical properties of the tread, and in the first place rolling resistance and abrasion resistance, that should be improved or maintained.
The technical problem underlying the present invention is that of providing a tire for vehicles provided with a tread including silica as reinforcing filler, which has a reduced rolling resistance, while keeping wear resistance and wet skid resistance substantially unaltered, and which allows at the same time to overcome the plant and process constraints associated to the need of reducing the projected area of the aggregates under 8400 nm2 in order to achieve the desired homogeneous dispersion of silica, according to the teachings of the aforementioned prior art.
According to the invention, the Applicant has surprisingly found that it is possible to achieve the desired optimum compromise between rolling resistance, wear resistance and wet skid resistance even by using a silica-based reinforcing filler having a low surface area thanks to a particular combination of average projected area of the aggregates as measured before and after incorporation into the polymer base and dispersibility of the same in the polymer base of the tread.
More specifically, the Applicant has found that a silica-based reinforcing filler having a low surface area may be thoroughly dispersed in the vulcanized polymer base of the tread of a vehicle tire while still showing a projected area of the aggregates that exceeds 8500 nm2, i.e. greaterxe2x80x94considering an average margin in the experimental measurement of the same of xc2x1100 nm2xe2x80x94than the aforementioned maximum limit of 8400 nm2 taught by the prior art.
Besides, the Applicant has surprisingly found that such silica-based reinforcing fillers having a low surface area may be homogeneously dispersed in the vulcanized polymer base of the tread, reaching the above value of the dispersion index also in the absence of an exceedingly high thermal-mechanical working of the rubber compositions.
According to a first aspect of the invention, the aforementioned technical problem is solved by a low rolling resistance tire for vehicles as defined in attached claim 1.
In the following description and in the appended claims, the term: xe2x80x9cdispersion indexxe2x80x9d, is used to indicate the percentage of dispersed silica particles with respect to the total number of silica particles included in the polymer base of the tread. Conventionally, the silica is considered as dispersed when it is present in particle aggregates having a diameter smaller than 7 xcexcm.
The Applicant has surprisingly found that by incorporating and homogeneously dispersing in the polymer base a silica having a particular combination of surface area, average projected area of the aggregates as measured before and after incorporation into the polymer base, it is possible on the one hand to further reduce the rolling resistance of the tire without adversely affecting wear resistance and wet skid resistance and, on the other hand, to reduce the thermal-mechanical working to be imparted to the rubber composition to obtain a good dispersion of said filler.
The Applicant has in fact found that in order to achieve the aforementioned advantageous effects it is not necessary to employxe2x80x94as constantly suggested by the prior artxe2x80x94particles having a high surface area and characterized by a reduced particle size and, as such, difficult to be homogeneously dispersed in the polymer base of the tread, but it is possible to effectively use silicas having a low surface area, characterized by particles having a greater size compared to silicas having a high surface area, and requiring with respect to the latter a lower thermal-mechanical working to achieve a suitable dispersion level in the polymer base of the tread.
Additionally, since during the incorporation step of the silica having a low surface area in the polymer base a temperature increase occurs that is lower than that reached by the processes and apparatuses of the prior art, the risks of thermally degrading the coupling agent of the silica are advantageously reduced, while imparting a strong thermal-mechanical action in the step of silica incorporation in the polymer base.
Preferably, the silica-based reinforcing filler has the following properties:
an average projected area of the aggregates as measured before incorporation into the polymer base comprised between 9000 and 18000 nm2 and, still more preferably, between 10000 and 15000 nm2;
a dispersion index not lower than 99% and, still more preferably, not lower than 99.5%;
an average projected area of the aggregates as measured after incorporation into the polymer base comprised between 9000 and 15000 nm2 and, still more preferably, between 10000 and 12000 nm2;
a BET surface area (measured according to ISO standard 5794/1) not greater than 130 m2/g and, still more preferably, not greater than 110 m2/g, a surface area measured by CTAB absorption according to ISO standards 6810 comprised between 80 and 130 m2/g and, still more preferably, comprised between 90 and 120 m2/g;
a mean aggregate size measured by photon correlation spectroscopy lower than 350 nm, more preferably between 120 and 250 nm and still more preferably, between 160 and 200 nm;
For the purposes of the invention, the silica-based reinforcing filler having a low surface area comprises silica obtained by precipitation, as disclosed, for instance, in European patent EP 0 157 703. Silica may possibly be aluminium doped, as disclosed, for instance, in European patent application EP 0 735 088.
The silica-based reinforcing filler having a low surface area of the invention, furthermore, may be in the form of powder, granules, extrudates or spherical beads.
The Applicant has found that readily-dispersible silica-based reinforcing filler of preferred use are silicas having a particular physical form, i.e. silicas in the form of substantially spherical beads.
In the following description and in the appended claims, the term: silica in the form of substantially spherical beads is used to indicate a silica essentially constituted by spheroidal particulates obtained by precipitation. Such a type of silica may also be designated in the art with the term micropearls or MP.
Suitable silicas of this type are available on the market and a precipitation process suitable to obtain silicas having the required physical form is disclosed, for instance, in European patent application EP 0 520 862.
Preferably, the silica in the form of substantially spherical beads has a mean particle size of at least 80 microns, more preferably between 80 and 300 microns and still more preferably, between 100 and 250 microns.
According to an alternative embodiment of the invention, the polymer base comprises a predetermined amount of a second silica-based reinforcing filler.
For the purposes of the invention, this second silica-based reinforcing filler may be silica having a high surface area of a type known per se, such as that disclosed, for instance, in European patent application EP-A-0 501 227.
Preferably, the second silica-based reinforcing filler has a BET surface area (measured according to ISO standard 5794/1) comprised between 130 and 200 m2/g, still more preferably between 150 and 180 m2/g, a surface area measured by CTAB absorption according to ISO standards 6810 comprised between 130 and 200 m2/g, still more preferably between 140 and 170 m2/g.
According to the invention, the total amount of silica-based reinforcing filler is comprised between 20 and 120 parts by weight per 100 parts by weight of polymer base.
Preferably, the total amount of silica-based reinforcing filler is comprised between 30 and 100 parts by weight per 100 parts by weight of polymer base, still more preferably comprised between 50 and 90 parts by weight per 100 parts by weight of polymer base.
According to the invention, if the polymer base comprises both types of silica-based reinforcing fillersxe2x80x94namely, low and high surface areaxe2x80x94the minimum amount of silica having a low surface area is preferably of at least 50% by weight of the total amount of silica-based reinforcing filler.
Preferably, the polymer base of the tread further comprises an additional reinforcing filler selected from those commonly used in the field, such as for instance: carbon black, alumina, aluminosilicates, calcium carbonate, kaolin and the like.
The types of carbon black conventionally used in the art and which may be employed in the tire tread of the invention, comprise those designated by the ASTM standards as N110, N121, N220, N231, N234, N242, N239, N299, N315, N236, N330, N332, N339, N347, N351, N358 and N375.
Preferably, the carbon black-based reinforcing filler has a DBP absorption value measured according to ISO standards 4656-1 of about 80 ml/100 g, and a surface area, measured by CTAB absorption according to ISO standards 6810, not higher than 80 m2/g.
For the purposes of the invention, such carbon black-based reinforcing filler is preferably comprised between 0 and 100 parts by weight per 100 parts by weight of polymer base, more preferably, between 0 and 40 parts by weight per 100 parts by weight of polymer base. The weight ratio between the total amount of silica-based reinforcing filler and the amount of additional reinforcing filler should be such as to ensure the desired low rolling resistance; in particular such ratio is preferably comprised between 0.5 and 15 and, still more preferably, between 1 and 10.
According to the invention, the rubber composition employed to manufacture the tire tread of the invention may further incorporate a suitable coupling agent, capable of interacting with silica and of binding the latter to the polymer base during the vulcanization of the same.
Coupling agents of preferred use are the silane-based ones and have the following structural formula:
(R)3xe2x80x94Sixe2x80x94CnH2nXxe2x80x83xe2x80x83(I)
wherein:
R is an alkyl or an alkoxy group comprising 1 to 4 carbon atoms or a chlorine atom, n is an integer of between 1 and 6, and X is a group selected from xe2x80x94Simxe2x80x94CnH2nxe2x80x94Sixe2x80x94(R)3, nitroso, mercapto, amino, epoxy, vinyl, imido, a chlorine atom, one or more sulfur atoms, or a SmY group, wherein Y is selected from the following functional groups: 
wherein m and n are an integer comprised between 1 and 6, and R is an alkyl or an alkoxy group comprising 1 to 4 carbon atoms or a chlorine atom.
Particularly preferred among them is the silane coupling agent Si69 [bis(3-triethoxysilyl-propyl)tetrasulphide] (DEGUSSA), as such or in a suitable mixture with a moderate quantity of inert filler (for instance carbon black or the same silica), so as to facilitate its incorporation into the rubber composition, or X50S (Degussa) (50% carbon black, 50% silane).
Preferably, the polymer base used for the manufacture of the tire tread includes unsaturated-chain polymers or copolymers, either natural or synthetic, the latter being obtained by solution or emulsion polymerization of conjugated dienes and aliphatic or aromatic vinyl monomers.
More particularly, the aforementioned polymer base comprises copolymers obtained by polymerization of at least one conjugated diolefin with at least one aromatic vinyl hydrocarbon, said copolymers having a glass transition temperature comprised between 0xc2x0 and xe2x88x9280xc2x0 C. and including a total amount of aromatic vinyl hydrocarbon comprised between 5% and 50% by weight based on the total weight of the same.
For the purposes of the invention, the conjugated diolefin of preferred use is selected from the group comprising:
1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, 1,3-hexadiene, and mixtures thereof, while the aromatic vinyl hydrocarbon of preferred use is selected from the group comprising: styrene, xcex1-methyl-styrene, p-methyl-styrene, vinyl-toluene, vinyl-naphthalene, vinyl-pyridine, and mixtures thereof.
Preferably, the aforementioned copolymers are obtained by solution polymerization of the conjugated polyolefin with the aromatic vinyl hydrocarbon. Still more preferably, the aforementioned copolymers are obtained by solution polymerization of 1,3-butadiene and styrene according to methods known per se.
For the purposes of the invention, the aforementioned polymer base may further comprise natural rubber, polybutadiene, polyisoprene, polychloroprene, possibly halogenated isoprene-isobutene copolymers, butadiene-acrylonitrile copolymers, styrene-butadiene-isoprene terpolymers, and ethylene-propylene-diene terpolymers.
In a particularly preferred embodiment, the polymer base includes at least 30% by weight, more preferably from 45 to 90% by weight of a butadiene/styrene copolymer prepared in solution (S-SBR), on the total dry weight of the polymer base.
In a particularly preferred embodiment, such a butadiene/styrene copolymer is of the so-called xe2x80x9chigh vinylxe2x80x9d type, i.e. wherein at least 50% by weight of the conjugated diolefin (butadiene) polymerizes in 1,2 form with the aromatic vinyl hydrocarbon (styrene) in such a manner as to have an amount of an olefin fraction having a 1,2 structure comprised between 30% and 70% by weight based on the total weight of the same.
In the following description and in the appended claims, the term: 1,2-polymerization, is used to indicate a particular stereospecific polymerization method between the conjugated diolefin and the vinyl aromatic hydrocarbon by means of which the formation of a copolymer is obtained in which the olefin fraction comprises a prefixed quantity of side-chain vinyl groups xe2x80x94CHxe2x95x90CH2 bound to the polymer chain.
Methods for the 1,2-polymerization of conjugated olefins are well known in the art and are described, for instance, in U.S. Pat. Nos. 3,451,988 and 4,264,753.
In an embodiment, at least one of the aforementioned copolymers is a copolymer suitably modified by means of silane groups Sixe2x80x94Oxe2x80x94having chemical and structural features, as well as preparation techniques known per se, such as for instance those disclosed by European patent application EP-A-0 447 066.
In this embodiment of the invention, the copolymer including silane groups has a glass transition temperature not lower than xe2x88x9250xc2x0 C., and is obtainable by polymerization in the presence of an organic metal initiator of 1,4-butadiene or a 1,4-butadiene/styrene copolymer with a silane compound having the following structural formula:
Xixe2x80x94Sixe2x80x94(OR)jxe2x80x94Rxe2x80x24-i-jxe2x80x83xe2x80x83(II)
wherein X is a halogen atom selected from the group comprising chlorine, bromine and iodine, R and Rxe2x80x2 are independently an alkyl group, an aryl group, a vinyl group or a halogenated alkyl group having from 1 to 20 carbon atoms, j is an integer between 1 and 4, i is an integer between 0 and 2, the sum of i and j ranging between 2 and 4.
Preferably, the aforesaid silane compound comprises non-hydrolytic OR groups, i.e., the OR group is a non-hydrolytic alkoxy, aryloxy or cycloalkoxy group having 4 to 20 carbon atoms. Preferably, the R radical in the OR group is a hydrocarbon residue wherein 3 carbon atoms are bonded to a carbon atom in xcex1-position, a hydrocarbon residue having not less than one carbon atom which is bonded in xcex2-position to a carbon atom or an aromatic hydrocarbon residue, such as for instance a phenyl or tolyl group.
Preferably, the aforesaid silane compound comprises non-hydrolytic OR groups, i.e. the OR group is a non-hydrolytic alkoxy, aryloxy or cycloalkoxy group having from 4 to 20 carbon atoms. Preferably, the R radical in the OR group is a hydrocarbon residue wherein 3 carbon atoms are bonded to a carbon atom in xcex1-position, a hydrocarbon residue having not less than one carbon atom which is bonded in xcex2-position to a carbon atom or an aromatic hydrocarbon residue, such as for instance a phenyl or tolyl group.
Among the alkoxy group-comprising silane compounds suitable for the purposes of the invention, those preferred are tetrakis(2-ethylethoxy)silane, tetraphenoxy silane, methyltris(2-ethylethoxy)silane, ethyltris(2-ethylethoxy)silane, ethyltrisphenoxy silane, vinyltris(2-ethylhexylethoxy)silane, vinyltriphenoxy silane, methylvinylbis(2-ethylhexyletoxy)silane, ethylvinylbiphenoxy silane, monomethyltriphenoxy silane, dimethyldiphenoxy silane, monoethyltriphenoxy silane, diethyldiphenoxy silane, phenyltriphenoxy silane, diphenyldiphenoxy silane, and the like.
Among the aryloxy group-comprising silane compounds suitable for the purposes of the invention those preferred are tetraphenoxy silane, ethyltriphenoxy silane, vinyltriphenoxy silane, dimethyldiphenoxy silane, monoethyltriphenoxy silane, diethyldiphenoxy silane, phenyltriphenoxy silane, diphenyldiphenoxy silane, and the like.
Suitable silane compounds comprising a halogen atom and a non-hydrolytic OR group with 4 carbon atoms comprise tri-t-butoxy-monochloro silane, dichloro-di-t-butoxy silane, di-t-butoxy-diiodo silane, and the like, while suitable silane compounds comprising a halogen atom and a non-hydrolytic OR group with 5 carbon atoms comprise triphenoxymonochloro silane, monochloromethyldiphenoxy silane, monochloromethylbis(2-ethylhexyloxy) silane, monobromoethyldiphenoxy silane, monobromovinyldiphenoxy silane, monobromoisopropenylbis(2-ethylhexyloxy) silane, ditolyloxydichloro silane, diphenoxydiiodo silane, methyltris(2-methylbutoxy) silane, vinyltris(2-methylbutoxy) silane, vinyltris(3-metylbutoxy) silane, tetrakis(2-ethylhexyloxy) silane, tetraphenoxy silane, methyltris(2-ethylhexyloxy) silane, ethyltriphenoxy silane, vinyltris (2-ethylhexyloxy) silane, vinyltriphenoxy silane, methylvinylbis(2-ethylhexyloxy) silane, ethylvinyldiphenoxy silane, and the like.
Suitable silane compounds comprising a halogen atom and an OR aryloxy group include triphenoxymonochloro silane, monochloromethyldiphenoxy silane, monobromoethyldiphenoxy silane, ditolyldichloro silane, diphenoxydiiodo silane and the like.
Among these silane compounds, those wherein i is 0 or 1, in particular tetraphenoxy silane and monomethyltriphenoxy silane, are preferred.
For the purposes of the invention, the silane compounds may be used either alone or in mixture with one another.
In addition to the above described ingredients, one or more non-cross-linking ingredients, known per se and necessary to impart the composition the necessary mechanical and workability characteristics, are incorporated in the rubber composition used to manufacture the tire tread of the invention.
In particular, such ingredients are selected from the group comprising plasticizers, working adjuvants, anti-oxidants, age-retarding agents, etc.
Furthermore, each of such ingredients is selected in amounts and ratios which may be easily determined by those skilled in the art.
The rubber composition is also caused to be cross-linkable by the addition and incorporation of a suitable vulcanizing agent, possibly and preferably associated to suitable activators and vulcanization accelerators.
The vulcanizing agent of most advantageous use if sulfur or sulfur-containing molecules (sulfur donors), with accelerators and activators well known to those skilled in the art.
Zinc compounds, and in particular ZnO, ZnCO3, zinc salts of fatty acids, either saturated on unsaturated, having 8 to 18 carbon atoms, such as for instance zinc stearate, directly formed in the rubber composition starting from ZnO and a fatty acid, as well as BiO, PbO, Pb3O4, PbO2, and mixtures thereof, have proved to be particularly effective.
According to a second aspect of the invention, a tread for vehicle tires is provided, in particular a premolded tread for covering worn tires having a low rolling resistance as defined in attached claim 13.
For the purposes of the invention, such tread comprises the ingredients and has the characteristics described above with reference to the tire according to the present invention.
Such tread is obtainable starting from a rubber composition including the aforementioned vulcanized polymer base, by means of processes and apparatuses known per se.
According to a further aspect of the invention, a method is provided for reducing the rolling resistance of a tire, said tire being provided with a belt structure coaxially extending around a carcass structure and a tread, coaxially extending around the belt structure and being externally provided with a rolling surface designed to get in touch with the ground, which is characterized in that the tire is provided with a tread having a low rolling resistance as defined above.